The present invention relates to a multipurpose keyboard using an electronic pressure sensing device and more particularly to a keyboard incorporating a digitizer pad featuring spatial minimization of a pressure contact area capable of locating the leading and trailing edges of a pressure contact area.
Digitizer pads and their functions are well known.
The United States patents to Eventoff et al., U.S. Pat. No. 4,739,299 issued April 19, 1988 and Margolin, U.S. Pat. No. 4,455,450 issued June 19, 1984 describe digitizer pads having opposing conductor pads with solid or interleaved conductor traces enabling the X--Y or XYZ coordinates of the point of force application against the digitizer pad to be identified. Such digitizer pads use a pointed instrument to apply force against the pad generating a severely localized pressure point.
However, when the localized contact point extends over a larger area where adjacent circuit traces are pressed into contacting relationships, ambiguities result due to the multiple points of pressure application. In such an instance, the measurement of the force position is approximately the barycentric average of the force or, EQU &lt;x&gt;=.intg.xF(x) dx/.intg.F(x)dx
where:
&lt;x&gt; is the measured force position PA1 F(x) is the distribution of force with position and the integrals extend along the entire length of the position sensor.
It is often desirable to measure the position of the leading or trailing edge of an object applying pressure, rather than its barycentric position. Specifically, a barycentric oriented digitizer pad would have disadvantages for use in an electronic keyboard. Measurement of the leading or trailing pressure edge is useful to determine the size of an object and further provides a dynamic means of measurement for an object's position in motion. For example, a person's fingers applied to a keyboard depressing the keyboard keys. A barycentric device could not distinguish between two keys being struck simultaneously, or a single key, midway between the former keys being struck.
An increasingly common method of building electronic keyboards for computers, word processors, telephones and industrial controls is the use of a membrane switch as the active sensing element.
This membrane switch may be covered with a simple printed template overlay or have mechanical key actuators over the membrane mimicking conventional keyboards. The membrane switch has an array of switches usually on a common substrate which are arranged in the desired keyboard pattern. Commonly, the key strokes are verified by a microprocessor which then translates the key strokes detected into a form suitable for a computer input bus.
It is advantageous to incorporate into a keyboard an electronic pressure sensing device such as a digitizer pad, wherein the essential element of this pad is the ability to detect more that one point of deflection on the pad simultaneously, because actuation element positions are not fixed at manufacture, and can be redefined by hardware or software means.
It is further desirous to utilize a digitizer pad that measures the leading or trailing edge of an object depressing the pad rather than its barycentric position.
A digitizer pad supporting a keyboard is described by U.S. Pat. No. 4,739,299 to F. N. Eventoff et al., issued April 19, 1988 entitled "Digitizer Pad". In general, the Eventoff et al. digitizer pad has generally an opposing conductor pad with solid or interleaved conductor traces to enable the X--Y or XYZ coordinates of the point force application against the digitizer pad to be identified.
Incorporating the standard XYZ digitizer pad as the type developed under U.S. Pat. No. 4,739,200, gives rise to two problems. First, key rollover cannot be detected occurs using this standard digitizer pad. Multiple key commands likewise cannot be detected. The Eventoff et al. XYZ digitizer produces a single point output detection corresponding to the center of an applied force distribution.
It is common for typists, especially typing at high speeds, to experience key rollover whenever a new key is struck before the last key is fully released. On a standard typewriter keyboard configuration, rollover between the letter "q" and the letter "t", for example, would be indistinguishable from striking on the "e" key. This first problem of key rollover can be avoided through the use of leading and trailing edge sensing properties of a digitizer pad operable to measure the vertical and/or lateral distance from the contact points to trailing edge.
A second problem associated with a standard XYZ digitizer pad is the inability to accommodate two key commands (e.g. shift-A for an upper case "A"). For example, on a standard keyboard layout, depressing "Shift" and "x" simultaneously would be indistinguishable from depressing the "z" key.
Finally, it is most desirous to construct a multipurpose keyboard which can receive a variety of template overlays thereby producing different functional keyboards without changing the digitizer pads contact area.